Method of and arrangement for co-combustion of biomasses and/or organic wastes as secondary fuel in a coal dust firing system

ABSTRACT

The invention concerns a method of and an arrangement for co-combustion of biomasses and/or organic wastes as secondary fuel in a coal dust-fired boiler of a vapor generator. The secondary fuel is dried directly or indirectly with the flue gas of the boiler. The vapors produced in the drying operation are fed to the combustion chamber of the boiler as final combustion air and/or secondary air.

The invention concerns a method of and arrangement for co-combustion ofbiomasses and/or organic wastes as secondary fuel in a coal dust firingsystem.

Such a method is known for example from DE 196 49 986 A1. The methoddescribed in DE 196 49 986 provides that sludge is introduced in theform of a suspension tangentially into a cyclone combustion chamber anddried and degassed on a spiral path with a heat transfer medium. Thefuel gas produced in that way, the vapors and the degassed fine sludgedust are fed to the combustion chamber and/or the flue gas suctionreturn system and/or an injection burner. The degassed coarse sludgedust is separated by centrifugal action from the cyclone combustionchamber and distributed to the carbon mill and/or the carbon burnerline.

That method is comparatively complicated and expensive in terms ofenergy technology, especially as the cyclone combustion chamber isoperated by means of an oil or gas burner.

DE 101 56 616 A1 discloses a method of co-combustion of sludge in afluidised bed boiler with a stationary or slightly expanding fluidisedbed, wherein mechanically dewatered, pumpable sludge is fed on to thefluidised bed in the fluidised bed combustion chamber of the fluidisedbed boiler immediately above the fluidised bed at a plurality oflocations over the periphery of the fluidised bed combustion chamber, bymeans of injection lances, and wherein the introduction is effected ineach case by means of compressed air so that scattering of the sludgeover the fluidised bed is effected. In the method described in DE 101 56616 the sludge introduced into the fluidised bed is also carried by thefluidised bed, mixed with the fluidised bed material and finallyundergoes joint combustion therewith.

Direct introduction of the sludge into the firing chamber of a boilerwithout previous treatment in that way, as is described by way ofexample in DE 196 49 986, is only possible in the case of a fluidisedbed firing system.

Organic materials which are to be introduced into a dust-fired boilerrequire preliminary drying depending on their respective moisturecontent. That applies in any case to mechanically dewatered sludges.

Biomasses and waste which have a comparatively high water content arebeing increasingly used in coal-fired power stations with dust firing.

DE 196 49 986 A1 already presupposes that the direct distribution ofmoist biomasses to coal dust mills is known. In fact moist biomasses andwaste are being increasingly distributed directly to the coal mills. Inthe case of a direct feed to the coal mills, only a limited amount canbe used as the drying reserves of the mills are limited and in mostcases the coal mills are also not suitable for comminuting moist,fibrous substances. Depending on the biomasses used, for example whendealing with moist wood, a drying operation is necessary prior to a finecomminuting step. A certain grain size of the biomasses to be used isnot to be exceeded because of a possible adverse effect on the dustfiring system.

Previous methods of co-combustion of biomasses and other organic wastesin coal dust firing systems therefore provide, as is also described inDE 196 49 986, for drying with natural gas or petroleum. That involvescomparatively high energy costs.

It is known from the drying of brown coal prior to the combustionthereof that the energy required for the drier is to be branched out ofthe water vapor circuit of the power station process. That is linked toa comparatively high level of apparatus complication and expenditure,which is not in any way related to the energy yield which can beattained when drying biomasses and other organic waste.

At any event post-treatment of the drier vapors and fumes is complicatedand expensive. Under some circumstances when condensation of the driervapors and fumes takes place effluent reprocessing is required.

Therefore the object of the invention is to provide a method of and anarrangement for co-combustion of biomasses and/or organic wastes in acoal dust firing system, with which the aforementioned problems areavoided.

To attain the object of the invention there is provided a method ofco-combustion of biomasses and/or organic wastes as secondary fuel in aboiler of a vapor generator, which is fired with coal in grain and/ordust form as fuel, wherein the secondary fuel is dried directly orindirectly with the flue gas of the boiler, and wherein the vapors fromthe drying of the secondary fuel are added to the combustion air fed tothe boiler.

An advantage of that operating procedure is that condensation of thevapors with downstream-disposed effluent reprocessing and possiblyconcomitant odor annoyance are not required.

In a preferred variant of the invention in which the combustion air isadded to the boiler in a stepped configuration over a plurality oflevels it is provided that the vapors are at least partially added inthe combustion air level which is last downstream in the flow directionof the flue gas.

To state that in different terms, the method according to the inventioninvolves co-combustion of moist biomasses and organic wastes assecondary fuel in a coal dust firing system, wherein the secondary fuelis pre-dried, the energy required for the drying operation is coupleddirectly out of the flue gas flow and the vapors produced in the dryingoperation—primarily water vapor—are added again to the flue gas still inthe boiler upstream in the direction of flow of the flue gas before theancillary heating surfaces. The dried secondary fuel can be allocated tothe coal mills and introduced with the primary air into the boiler.

Such coal mills can be for example in the form of known roller bowl-typemills in which the energy of the flue gas is used for pre-drying thecrushing material and the flue gas is used for transporting thatmaterial.

Desirably the addition of the vapors to the combustion air is effectedin dependence on the demand for output power of the drier for thesecondary fuel in such a way that optionally a partial amount of thevapors is added to the next combustion air level which is disposedupstream in relation to the flue gas. Desirably the amount of airrequired for the drier for the secondary fuel corresponds to the designamount for the last combustion air level in the downstream direction. Ifthe demand for output power of the drier for the secondary fuel ishigher than the air required in the last downstream combustion airlevel, for example in the form of final combustion air, cascade-likedistribution of the vapors can be effected in a direction towards thenext combustion air levels disposed in the upstream direction.

In a preferred embodiment of the method according to the invention theenergy required for the drier for the secondary fuel is coupled out ofthe flue gas flow downstream of an air preheater.

Desirably in that case a partial amount of the air flow is branched offdownstream of the air preheater for drying the secondary fuel. Anotherpartial amount of the air flow can be fed to the boiler downstream ofthe air preheater as primary and/or secondary air.

The primary air which is branched off downstream of the air preheater isdesirably fed to the boiler by way of at least one coal mill. There theenergy thereof is used for drying the primary fuel.

In a preferred variant of the method according to the invention finalcombustion air is added to the combustion air level which is last in thedownstream direction. The vapors of the drier can be completely suppliedas the final combustion air. In the case involving a stepped combustionair feed without the addition of final combustion air, the vapors can befed to the boiler as secondary air.

Preferably direct drying is effected in the air flow in the drier forthe secondary fuel.

As an alternative thereto it is possible that indirect drying iseffected in the drier for the secondary fuel.

Compression and removal of dust from the vapors can be provideddownstream of the drier. In that case compression serves to compensatefor the pressure losses of the drier.

The object of the present invention is further attained by anarrangement for co-combustion of biomasses and/or organic wastes assecondary fuel in a coal dust firing system, comprising at least onedrier for the secondary fuel, wherein the drier is connected downstreamof an air preheater for conditioning of combustion air and/or mill airand wherein the vapors outlet of the drier is connected to the finalcombustion air feed and/or the secondary air feed of the boiler.

The invention is described hereinafter by means of three embodiments byway of example illustrated in the drawings in which:

FIG. 1 shows an arrangement of a dust-fired boiler with air preheaterand drier for secondary fuel in accordance with a first embodiment ofthe invention,

FIG. 2 shows a diagrammatic view of a method implementation modified incomparison to the FIG. 1 embodiment in accordance with a secondembodiment of the invention, and

FIG. 3 shows a diagrammatic view of a third embodiment of the invention.

The embodiment illustrated in the drawings has a dust-fired boiler 1which is operated for example with hard coal as primary fuel. The coalwith which the boiler 1 is operated is ultimately not critical to theinvention, it can equally be fired with brown coal.

Of the boiler 1, only the combustion chamber is illustrated, the upperregion of the boiler with the ancillary heating surfaces not being shownfor the sake of simplicity in the flow chart. The boiler 1 is in theform of a boiler with stepped combustion air feed. The individualcombustion air levels are denoted by references 2 through 6, wherein thelowermost combustion air level 2 represents the infiltrated air feed tothe combustion chamber, the following combustion air level 3 representsthe primary air feed, the combustion air level 4 disposed downstreamthereof represents the secondary air feed, the combustion air level 5 inadjoining relationship downstream in relation to the flow gas representsthe upper air feed and the last downstream combustion air level 6represents the final combustion air level. Reference 7 denotes the fluegas flow out of the combustion chamber or the boiler 1.

As can be seen in particular from FIG. 1 the flue gas flow is passed byway of an air preheater 8 in the form of a rotary air preheater—referredto hereinafter as the preheater. The preheater 8 is connected in knownmanner to a fresh air feed 9. The fresh air is heated in the preheater 8directly or indirectly with flue gas from for example 35° C. to about350° C. At about 350° C. the heated air flows through the drier 10 inwhich biomasses in the form of for example moist wood chips or the likeare dried in direct contact with the heated air.

The vapors outlet 11 of the drier 10 is directly connected to thecombustion chamber of the boiler 1. In the described embodiment 100% ofthe vapors is fed to the boiler 1 as final combustion air 6. The driedmaterial withdrawn from the drier is allocated to the coal mills (notshown) which are arranged in the primary air feed 3 to the combustionchamber.

Alternatively the biomasses can be ground downstream of the drier 10 ina separate mill and after the grinding operation injected separately orjointly with the coal into the boiler.

In accordance with a further variant it can be provided that the biomassis crushed prior to the drying operation in a separate mill. Finally thebiomasses can already be present in comminuted form so that additionalgrinding is unnecessary.

Whether separate grinding of the biomasses is required optionally priorto or after the drying operation is predominantly a question of thephysical properties of the biomass in regard to grindability thereof.

When injecting the biomasses jointly with the coal it is appropriate forit to be allocated to the coal mills in which intensive thorough mixingof biomass and coal and possibly also post-crushing of the biomass areachieved.

Downstream of the preheater 8 and upstream of the drier 10 heated air isdistributed in known manner by way of the combustion air levels of thecombustion chamber of the boiler 1, wherein the primary air feed 3 ispassed in known manner by way of one or more coal mills which are notillustrated here.

Regulation provides that, with an increased demand for output power ofthe drier 10, a partial amount of the vapors is added cascade-like tothe upper air 5 and possibly also the secondary air 4.

By way of the final combustion air feed 6 and/or the upper air feed 5and/or the secondary air feed 4, the vapors pass out of the drier 10into the flue gas flow 7 and finally into the flue gas outlet 12. Tocompensate for any pressure losses in the drier 10, a vapors compressor13 is connected downstream of the drier 10. A filter can also beconnected downstream of the vapors compressor 13, if required, such afilter is not shown in the simplified view of the operating procedure ofthe method.

The embodiment illustrated in FIG. 2 approximately corresponds to thatof FIG. 1, with the difference that the drier 10 is in the form of anindirect drier and has two outlets 11 a, 11 b, the outlet 11 a servingas a vapors outlet and the outlet 11 b in contrast passing the cooledheat exchange medium of the drier 10. The outlets 11 a and 11 b arebrought together again upstream of the vapors compressor 13.

In other respects identical parts are denoted by the same references inthe various Figures.

That also applies to the embodiment which is illustrated in FIG. 3 andwhich differs from the preceding embodiments insofar as the drier 10 isin the form of an indirect drier which however has a separate circuit 15for the heat exchange medium. That heat exchange circuit 15 is closed initself. Provided therein is a compressor 16 in order to produce thecirculatory flow of the heat exchange medium.

Provided for heating the heat exchange medium is a heat exchanger 17,the outlet 11 b of which is combined downstream of the drier 10 with thevapors outlet 11 a of the drier 10.

In the embodiment described with reference to FIG. 2 by way of examplethe drier 10 could be in the form of a fluidised bed drier with inwardlydisposed heating surfaces.

In the embodiment described with reference to FIG. 3 it would bepossible to provide for example vapor, nitrogen or flue gas as the heatexchange medium. The cooled air from the outlet 11 b of the heatexchanger 17 is added together with the vapors to the final combustionair or introduced into the combustion chamber of the boiler 1 as a finalcombustion air feed.

LIST OF REFERENCES

-   1 boiler-   2 infiltrated air feed to the combustion chamber-   3 primary air feed to the combustion chamber-   4 secondary air feed to the combustion chamber-   5 upper air feed to the combustion chamber-   6 final combustion air feed to the combustion chamber-   7 flue gas flow-   8 air preheater-   9 fresh air feed to the air preheater-   10 drier-   11 vapor outlet-   11 a vapors outlet-   11 b drier outlet/heat exchanger outlet-   12 flue gas outlet-   13 vapors compressor-   14 secondary fuel flow-   15 heat exchanger circuit-   16 compressor-   17 heat exchanger

1. A method of co-combustion of biomasses and/or organic wastes assecondary fuel in a boiler of a vapor generator, which is fired withcoal in grain and/or dust form as primary fuel, wherein the secondaryfuel is dried directly or indirectly with the flue gas of the boiler,and wherein the vapors from the drying of the secondary fuel are addedto the combustion air fed to the boiler.
 2. A method as set forth inclaim 1 wherein in which the combustion air is added to the boiler in astepped configuration over a plurality of levels characterised in thatthe vapors are at least partially added in the combustion air levelwhich is last downstream in the flow direction of the flue gas.
 3. Amethod as set forth in claim 1 characterised in that the addition of thefuel to the combustion air is regulated in dependence on the demand foroutput power of the drier for the secondary fuel in such a way thatoptionally a partial amount of the vapors is also added to the nextcombustion air level which is disposed upstream in relation to the fluegas.
 4. A method as set forth in claim 2 characterised in that theamount of air required for the drier for the secondary fuel correspondsto the design amount for the last combustion air level in the downstreamdirection.
 5. A method as set forth in claim 1 characterised in that theenergy required for the drier for the secondary fuel is coupled out ofthe flue gas flow downstream of an air preheater.
 6. A method as setforth in claim 5 characterised in that a partial amount of the air flowis branched off downstream of the air preheater for drying the secondaryfuel.
 7. A method as set forth in claim 5 characterised in that apartial amount of the air flow is fed to the boiler downstream of theair preheater as primary and secondary air.
 8. A method as set forth inclaim 7 characterised in that the primary air which is branched offdownstream of the air preheater is fed to the boiler by way of at leastone coal mill.
 9. A method as set forth in claim 2 characterised in thatfinal combustion air is added to the combustion air level which is lastin the downstream direction.
 10. A method as set forth in claim 1characterised in that direct drying is effected in the air flow in thedrier for the secondary fuel.
 11. A method as set forth in claim 1characterised in that indirect drying is effected in the drier for thesecondary fuel.
 12. An arrangement for co-combustion of biomasses and/ororganic wastes as secondary fuel in a coal dust firing system,comprising at least one drier for the secondary fuel, wherein the drieris connected downstream of an air preheater for conditioning ofcombustion air and/or mill air and wherein the vapors outlet of thedrier is connected to the final combustion air feed and/or the secondaryair feed of the boiler.